A system for estimating a long downhill run in a car of one's own is disclosed in Japanese laid-open Patent Publication No. 2001-122092. The system estimates that the car is running just now on a long downhill when the following accumulated count value reaches a preset value. The accumulated count value is obtained by the following way using computing means: adding a weighting factor to time in which an exhaust brake or a retarder is operated in an “on” state and accumulating the resultant time as accumulated braking time; on the other hand, adding a weighting factor to time in which an accelerator pedal is operated and accumulating the resultant time as accumulated accelerating time; and obtaining the accumulated count value by subtracting from the accumulated braking time to the accumulated accelerating time. The above-mentioned prior art is system for estimating whether the road on car's running is a downhill or not but does not obtain a downhill gradient of the road.
When a car with an engine controller having no atmospheric pressure sensor descends a long downhill, fuel cut is continuously performed, and a throttle is usually in a fully closed state, but the atmospheric pressure value cannot be updated. In this case, when controlling an electronic control throttle valve in accordance with the balance on torques of a demand torque based on an accelerator pedal position and a motor generator torque or the like, the atmospheric pressure value is remained at high altitude. Therefore, when the fuel cut is reset to fuel supply by a driver's accelerator pedal operation after a downhill running of the car is fished, the throttle position is slightly wider than the true throttle position. Consequently, until the atmospheric pressure value is updated again by a tune-up on lowland, acceleration is excessive with respect to the accelerator position operated by the driver, so that drivability deteriorates.